Fire fighting equipment and processes are an essential part of public safety and protection of property. Fire fighting departments are organized under city, county, and private companies and brigades. The fire fighting departments use a variety of equipment, and provide training to fire fighters in proper use of such equipment in fighting fires, fire prevention, and public safety.
Fire fighting equipment is often classified by the type of flammable material which it is most effective against. Class A fires and related equipment involve solid combustibles, building materials, structures, rubbish, vehicles, industrial, marine, wildlands, and the like. Class B fires relate to flammable liquids, Class C fires are electrical fires, and Class D fires involve combustible metals. Water alone is often not the most efficient and effective fire-extinguishing agent. Water addresses only the heat portion of the heat-fuel-oxygen fire interaction. In many situations, Class A foam mixed with water is more effective in extinguishing the flames. Class A foam contains a surface active agent, which reduces the surface tension of the water, allowing it to penetrate into the fuel surface. The foam bubbles cling to the fuel surface, isolating the fuel from the heat and oxygen. The water droplets in Class A foam are smaller than in a conventional water fog spray pattern, which provides for a more rapid conversion to stream when applied to a fire, resulting in better heat absorption.
The water and foam combination must have the proper mixture or percent concentration of foam in the water stream. The water has a flow rate as determined by the pressure and diameter of pipe. The water further has a certain conductivity based on the mineral, foreign matter, or particulate content, also known as hardness, of the water from its source. The foam is pumped from a tank or reservoir and injected into the water stream. The flow rate of the foam must proportionately match the flow rate of the water stream and take into account the conductivity of the water source in order to produce an effective foam concentration in the water stream as projected onto the fire.
Conventional electronic direct injection foam proportioning equipment is based on a volumetric approach using the water flow rate as measured by a turbine-flow meter for the foam delivery system. The foam concentrate flow rate is adjusted either manually or automatically to the desired percentage of the water flow. The foam is introduced into the water stream according to the water flow rate.
However, there exist a number of variables in the various electronic direct injection foam delivery systems that can lead to an inaccurate ratio of foam concentration in the water stream as projected onto the fire. Volumetric flow-based electronic foam proportioners do not automatically adjust for varying water hardness, which affects the quality of the finished foam mixture. The volumetric foam proportioners also do not automatically or accurately adjust for the variation in the detergent strength of the commercially available foam concentrates, which also affects the quality of the finished foam. Some utilize a motor-mounted velocity feedback sensor, which may not accurately represent the actual foam concentrate flow. The velocity of the water flow rate and the foam concentration in the water stream are in fact independent variables, which relate only when the system is working perfectly. The foam pump could even run dry or pump the wrong liquid and the proportioner will continue to function as though it were operating correctly.
In some situations, e.g., when responding to a large fire, there may not be a fire hydrant in proximity to the blaze or, due to inadequate water pressure, it may be necessary to tap into supplemental water sources to provide the necessary flow to extinguish the fire. Water may be supplied from an alternate source such as a water tanker or drafted from a nearby body of water. The water stored in the truck's tanks or siphoned from the body of water may not have the same conductivity characteristics as the water available from the hydrant water system. Moreover, the conductivity of water is known to vary from location to location. Variation in water conductivity will likely lead to incorrect foam concentration or foam effectiveness in the water stream as projected onto the fire.